For aviation platforms, the goal is to design the highest power density system while exceeding the required reliability standards. For aircraft pumps, thermal efficiency is especially important due to the added role of fuel and oil being heat sinks for various subsystems. Aircraft pumps are typically mounted to and driven by the accessory drive gearbox, which in turn is driven by the high-pressure gas turbine engine spool. Since the accessory engine gearbox has a constant gear ratio, the aircraft pump input rotary speed is directly related to engine spool speed.
Pump efficiency is maximized when the fluid displacement of a pump matches the particular demand requirement of the engine and associated subsystems. To accomplish this, various attempts have historically been made by others to improve pump efficiency by employing variable displacement pumps coupled with various valving arrangements.
Today's variable displacement pumps typically vary the fluid pumped per revolution by varying the stroke of the pumping element, such as a piston in a piston pump or a vane in a vane pump. Another technique that is employed is a multi-stage pump that has the capability to “unload” or switch a stage on and off.
The fore-mentioned systems do improve pump thermal efficiency but at the expense of increased weight and cost. For instance, an actuation system is required to move a cam so that the stroke of a vane pump can be varied, and in a multi-stage pump, two sets of pumping elements are required as well as special valving to unload a stage. Additionally, when a pumping stage is unloaded the “windage” and “churning” energy losses are still present due to the higher than required pump input speed.
Embodiments disclosed include a variable speed drive that allows a high horsepower, gearbox driven pump to behave like a motor driven pump without the weight penalty induced by a high horsepower motor. This drive is capable of setting and maintaining the pump rotary speed independent of the external gearbox speed so that the pump can deliver the required flow displacement for any given flight condition. Other advantages include the capability to constantly operate an automotive alternator at its best efficiency speed no matter what the engine speed is. These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.